1. Field of the Invention
The present invention relates to a boot, which covers a constant velocity universal joint being indispensable to a joint for a driving shaft of a front-wheel-drive vehicle, and which inhibits water and dust from intruding into a joint unit of such a constant velocity universal joint.
2. Description of the Related Art
A joint unit of a constant velocity universal joint has been conventionally covered with a bellows-shaped boot. The boot inhibits water and dust from intruding, thereby enabling the constant velocity universal joint to keep rotating smoothly at a large joint angle. The boot for a constant velocity universal joint includes a major-diameter cylindrical member, a minor-diameter cylindrical member, and a bellows member. The major-diameter cylindrical member is held to a joint outer race, etc., and has a major diameter. The minor-diameter cylindrical, member has a diameter smaller than that of the major-diameter cylindrical member, and is held to a shaft. The bellows member connects the major-diameter cylindrical member with the minor-diameter cylindrical member integrally, and is formed as a truncated cone shape substantially. When the boot is put in service, the bellows member deforms in accordance with the variation of the angle, which is formed between the joint outer race, etc., and the shaft, (i.e., the joint angle). Accordingly, even when the joint angle enlarges, it is possible to securely seal the joint unit by the boot.
Moreover, in the bellows member of the conventional boot for a constant velocity universal joint, crests and roots have been formed alternately, and the respective crests and roots have usually been formed as similar shapes.
Incidentally, when the boot is put in service by installing it to a constant velocity universal joint, and when the constant velocity universal joint is rotated while it is bent at a large joint angle, there arises a case where the side surfaces of the neighboring crests of the bellows member are pressed heavily against with each other so that abnormal slipping sounds (hereinafter referred to xe2x80x9cabnormal noisesxe2x80x9d) are generated.
The cause of the abnormal noises is believed as follows. When a constant velocity universal joint rotates while it is bent, the boot starts rotating while the crests, which contact with each other, are stuck closely to each other in a case where the surface of the bellows member exhibits a large friction coefficient. However, since the peripheral lengths of the crests, which neighbor with each other, vary, there arise differences in the peripheral direction. As the rotation develops, the differences enlarge. Then, at a time when the repulsive forces, which result from the deviations, are greater than the sticking forces, the crests are separated from each other so that the stresses are canceled. The surface of the bellows member is cyclically fluctuated by the repetitions of the sticking and separation. The fluctuation results in the vibrations of the air around the bellows member. Thus, the abnormal noises are generated.
Actually, water is present on the surface of the bellows member. In such a case, the friction coefficient is reduced remarkably by the intervention of water. However, the portions, on which water exists, and the portions, on which water does not exist, coexist inevitably. Accordingly, the friction coefficient is considerably fluctuated locally so that the bellows member vibrates more greatly. Thus, the abnormal noises are generated even more harshly.
Therefore, in order to inhibit the generation of the abnormal noises, it is effective to reduce the friction coefficient of the entire bellows member. Accordingly, it is possible to think of forming at least the bellows member of a thermoplastic elastomer, which contains a wax, an oil, or the like. However, in order to effectively suppress the abnormal noises, it is necessary to include a wax, an oil, or the like, in a large amount in a thermoplastic elastomer. If such is the case, there arise drawbacks in that the bellows member is lowered in terms of the fatigue resistance, and in that it is deteriorated in terms of the moldablility. Moreover, it is not possible to solve the problem with regard to the generation of the abnormal noises, which result from the intervention of water.
Hence, in Japanese Unexamined Patent Publication (KOKAI) No. 11-130,952, for example, a boot for a constant velocity universal joint is disclosed, boot which is formed of a thermoplastic polyester elastomer in which at least one compound selected from the group consisting of polyethers and non-denatured polyolefines is compounded with respect to a polyester block copolymer. When the bellows member is formed of such a special resin, the compounded compound leaches out on the surface of the bellows member, and accordingly it is possible to suppress the abnormal noises, which result from the intervention of water. Moreover, since the compound exhibits a good affinity with respect to water, water is not completely excluded by the pressures, which are exerted by the surfaces of the crests contacting with each other. Thus, water is put into a state that it intervenes in-between entirely so that the abnormal noises are inhibited from generating.
In addition, in order to configurationally reduce the abnormal noises, it is a preferable means to enlarge the shape of the crests or to reduce the peripheral length differences between the crests, which neighbor with each other. With such an arrangement, since it is possible to reduce the pressures, which are exerted by the surfaces of the crests contacting with each other, and since it is possible to reduce the differences in the peripheral direction during the rotation, it is possible to suppress the abnormal noises.
Incidentally, it has been recently required to lightweight vehicles. As the size of vehicle bodies has been made compact, it has been required to make the shape of boots for constant velocity universal joints compact.
In order to make boots for constant velocity universal joints compact, it is necessary to reduce the outside diameter of the bellows members. When the outside diameter of a bellows member is reduced, the film length of the crests is shortened by such an extent. Accordingly, in order to keep the displacement of the bellows member and the stress generated at the bellows member equivalent to those of conventional bellows members, it is necessary to enlarge the depth of the roots. However, when the depth of the roots is simply enlarged, there arises a case where the second root, which is designated at the second one from the side of the major-diameter cylindrical member, is bitten between the joint outer race and the shaft when the joint angle, which is formed between the central axis of the joint outer race and the central axis of the shaft, is enlarged. When the root is bitten in such a manner, a large force acts to the root. When the action occurs repeatedly, the sealing property is impaired because of damage to the root. Therefore, the boot must be frequently replaced. Thus, the life of such a boot is short.
Moreover, reducing the outside diameter of a bellows member entirely results in enlarging the pressures, which are exerted by the surfaces of the crests contacting with each other, during the rotation. Thus, such a measure affects adversely in the suppression of the abnormal noises.
The present invention has been developed in view of such circumstances. It is therefore an object of the present invention to solve the contradictory phenomena, suppressing the abnormal noises during the rotation and making the shape compact.
The present invention can achieve the aforementioned object, and comprises: a minor-diameter cylindrical member being held to a shaft; a major-diameter cylindrical member being separated from the minor-diameter cylindrical member, being disposed coaxially with the minor-diameter cylindrical member and having a diameter larger than that of the minor-diameter cylindrical member; and a bellows member connecting the minor-diameter cylindrical member with the major-diameter cylindrical member integrally, being substantially formed as a truncated cone shape, being formed of a thermoplastic elastomer exhibiting a kinetic friction coefficient of less than 0.6 when water intervenes in-between and including crests, in the cross section of the bellows member involving the central axis, the peaks of the crests, excepting the crest neighboring the major-diameter cylindrical member as well as the crest neighboring the minor-diameter cylindrical member, being disposed on an inner diametric side with respect to a line connecting the peak of the crest neighboring the major-diameter cylindrical member with the peak of the crest neighboring the minor-diameter cylindrical member.
It is desired that at least the bellows member can be formed of a thermoplastic elastomer including a compound exhibiting a high affinity to water as well as a low friction coefficient and being capable of leaching out on the surface of the bellows member.
Further, it is desired that, when the crest, neighboring the major-diameter cylindrical member, has an outside diameter of S1, and the crest, next to the crest, has an outside diameter of S2, a two-crest compact ratio, expressed by xe2x80x9cS2/S1,xe2x80x9d can be 0.77xe2x89xa6xe2x80x9cS2/S1xe2x80x9dxe2x89xa60.85.
Furthermore, it is desired that, when the bellows member is cut by a plane involving the axis, a line, connecting the peaks of the crests, can be formed as an arc shape having a radius xe2x80x9cRxe2x80x9d of from 100 to 200 mm.
Moreover, it is desired that the bellows member can include a plurality of crests and roots, at least a first root, a first crest, a second root, a second crest, a third root and a third crest, being disposed alternately in this order from the side of the major-diameter cylindrical member, and the outside diameter of the first crest can be substantially identical with the outside diameter of the major-diameter cylindrical member, and, in the cross section when the bellows member is cut by a plane involving the axis, the length of a line, extending from the peak of the first crest, passing over the second root and connecting the peak of the first crest with the peak of the second crest, can be substantially equal to the length of a line, extending from the peak of the second crest, passing over the third root and connecting the peak of the second crest with the peak of the third crest, and, when the second root has a depth of xe2x80x9chxe2x80x9d and the third root has a depth xe2x80x9cH,xe2x80x9d the depth xe2x80x9chxe2x80x9d can fall in a range of xe2x80x9cHxe2x80x9d/1.3xe2x89xa6xe2x80x9chxe2x80x9dxe2x89xa6xe2x80x9cHxe2x80x9d/1.1.
Namely, in accordance with the boot according to the present invention for a constant velocity universal joint, it is possible not only to make the shape compact but also to inhibit the abnormal noises from generating. Accordingly, it is possible to lightweight vehicles and to improve the silence in vehicles.